No experience with those electric heaters so I can't comment on that.
I really think in a car like a Seven you should try to heat the driver/passenger and not the car itself. There's just too much air/buffeting to contend with. I've had very good luck driving this season with no top and doors by using Gerbing heated clothing. I have a heated jacket liner that I wear under a hooded sweatshirt or leather jacket, along with a balaclava and insulated gloves and I've been able to continue driving comfortably with temps dropping down into the 20's. It really is surprising how well it works. Sometimes my left (outside) hand will start to get a little cool because of the airflow so I'll just rest it on the muffler to warm it up a little! If I decided to install the doors my hands would never get cold, or if they did I could get some of the heated gloves that plug into the jacket. You can check them out here: [/url]http://gerbing.com/
Lots of strange looks, but I'll still be driving the Birkin until the first ice/snow falls and they start salting the roads.